Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
The most common method of converting solar energy into electricity entails mounting standard photovoltaic modules on rooftops or on ground mounted structures. On average this approach turns sunlight into electricity at well under 20% efficiency.
Multi-junction PV cells are around 150% more efficient than these standard PV cells but they only operate optimally when sunlight is concentrated by lenses, prisms or mirrors and when the arrays are turned to face the sun very accurately by using highly accurate dual-axis tracking machines.
Using such trackers contributes around an additional 30-40% more electricity compared to fixed standard modules so it would be reasonable to expect over 3 times more power using trackers and multi-junction cells in combination. In practice however these gains are rarely realized because most concentrators require a very high level of tracking accuracy which is difficult to achieve. The angle of acceptance required is usually well under +−1.5 degrees and inaccurate tracking, even slightly outside of this small range, causes electrical outputs to virtually shut down. Quality control in the manufacturing of concentrator components is also proving difficult.
There have been a number of concentrators in the prior art in which concentrated light is coupled with a planar light guide after reflecting off vertically or horizontally stepped reflective faces. One design incorporates both horizontally and vertically stepped reflective faces. These devices usually step down or across by the depth or width of the injector element whereby the light guide rapidly increases in depth (where steps are in the vertical plane) or in width (where steps are in the horizontal plane). These arrangements restrict the potential lengths of the arrays, and therefore the area of light capture as well as the potential concentration ratio.
In the case of a horizontally stepped light guide (as depicted in FIGS. 1A and 1B), there are only a limited number of steps, each by the width of one injector element, before the primary concentrator elements in one light guide impinge on the space of concentrator elements of the adjacent light guides. In the case of a vertically stepped light guide (as depicted in FIG. 1C), the increasing depth becomes unmanageably thick as well as costly in terms of the volume of optical material required.
There are also devices that have ramped or tapered coupling elements. Some of these have the tapered elements set close to the bottom of a relatively thick light guide, meaning acceptance angles and/or concentration ratios are not high. Others use a series of tapered coupling elements in the same light stream where inevitably, downstream couplers interfere with light rays from upstream couplers and cause light to be decoupled from the light guide. These designs can cause significant losses of light as it decoupled and exits the system.
It would be advantageous if a device could be provided that addressed the above described problems of the prior art and which could direct concentrated sunlight onto multi-junction cells. It would be desirable that in at least preferred embodiments such a device had a much higher tolerance to tracking inaccuracy. Also it would be advantageous if embodiments of the device had characteristics of superior heat dissipation and uniform scattering of light rays over the entire surface of the cell and which was also suited for fabrication into large modules.